1. Field of the Invention
The present invention relates to a light-receiving module for use in the field of optical communication or optical information processing for receiving signal light from an optical fiber, and in particular, to a light-receiving module for receiving signal light amplified by a fiber optic amplifier.
2. Description of the Related Art
Light-receiving modules are provided at the end of optical fibers for receiving and converting signal light from the optical fibers into electric signals. A light-receiving module of the conventional type is composed of a lens 53 and photo-detecting device 51 and is housed in a case 50 as shown in FIG. 1. An avalanche photo diode (APD) or a pin photo diode is used for the photo-detecting device 51, which converts a light signal into an electric signal. The photo-detecting device 51 is optically coupled with optical fiber 52 through lens 53 in such a manner that light coming into the light-receiving module from optical fiber 52 will reach photo-detecting device 51.
When light signals are transmitted through the optical fiber, the intensity of the light signals attenuates in proportion to the distance transmitted due to transmission losses of the optical fiber. In order to overcome this problem in fiber optic communication systems now in use, repeaters are provided at fixed intervals for amplifying attenuated light signals. However, since repeaters are constructed so as to convert light signals into electric signals, amplify the electric signals and then convert them back into light signals for transmission, conventional repeaters are inevitably large-scale and expensive.
In order to solve these problems, recent research by technicians involved in optical amplification has resulted in the development of systems utilizing rare-earth doped fibers. Optical amplification is a method in which incident signal light is amplified in a light state for obtaining outgoing signal light without converting light signals into electric signals. In optical amplification using rare-earth doped fibers, rare-earth cations in the fiber are excited in advance by laser light of a wavelength in a range other than that of the signal light. In this manner, stimulated emission by the signal light occurs, thereby achieving light amplification. For example, an optical amplifier using an optical fiber doped with Neodymium (Nd) or Erbium (Er) is disclosed in the specification of British Patent GB 2,175,766A. An optical amplifier of this type will be hereinafter referred to as a fiber optic amplifier.
When a fiber optic amplifier is used, particularly as a preamplifier in a receiving station, it is necessary to reduce noise by removing spontaneous emission light emitted by rare-earth doped fibers. For this purpose, it is necessary to insert a bandpass filter of a narrow bandwidth which transmits only signal light to the emission side of the fiber optic amplifier. In FIG. 2, the structure of an example receiving station using a fiber optic communication system is shown which includes a fiber optic amplifier.
The fiber optic amplifier 60 is composed of a rare-earth doped fiber 54, a semiconductor laser module 55 which serves as a light source for exciting rare-earth ions in the rare-earth doped fiber 54, a coupler 56 provided in the incident side of the rare-earth doped fiber 54, and a fiber-type bandpass filter 58 provided at the emission side of the rare-earth doped fiber 54. The coupler 56 is used for transmitting both signal light from the optical fiber 59 coupled to the transmitting station and excitation light from the semiconductor laser module 55 into the rare-earth doped fiber 54. The emission side of the rare-earth doped fiber 54 is coupled to the optical fiber 52 by way of the bandpass filter 58, and the light-receiving module 57 is coupled with the other end of the optical fiber 52. Since the bandpass filter 58 has to transmit only the wavelength of a specific signal light and reduce other spontaneous emission light to a minimum, it must allow setting of its bandwidth of transmission wavelength to as narrow a range as possible (not more than several nanometers) with a highly precise center wavelength of transmission (not more than several tenths of a nanometer).
The structure of an example fiber-type bandpass filter 58 to be inserted into an optic transmission line is shown in FIG. 3. This bandpass filter 58 comprises two lenses 61, 62 which optically couple a rare-earth doped fiber 54 and an optical fiber 52 connected to the receiving side, and a bandpass filter plate 63 which transmits only a specific wavelength provided in the optical path of both lenses 61, 62. The bandpass filter plate 63 used is made by forming a dielectric multilayer film on the surface of a glass plate. The transmission center wavelength is usually set by adjusting the angle of incidence of light entering the bandpass filter plate 63.
For this bandpass filter 58, it is necessary that the two optical fibers 52 and 54 typically having cores 10 .mu.m in diameter be coupled in an optically efficient manner while the incident angle of light entering the bandpass filter plate 63 is adjusted, and it is therefore difficult to match the optic axes of the two optical fibers. Coupling losses due to the two lenses 61, 62 in the bandpass filter 58 cannot be disregarded, if an overall light amplification gain of the fiber optic amplifier 60 is to be achieved.
The bandpass filter 58 allows the center wavelength of transmission to be changed at will by changing the incident angle of incoming light by rotating the bandpass filter plate 63. However, since light enters the bandpass filter plate 63 at an oblique angle, the transmitted light beam shifts in a parallel direction due to the difference of refractive index between the bandpass filter plate 63 and air. In a system in which the optical fibers 52, 54 are mutually coupled, dimensional tolerance for parallel movement of the transmitted beam through the bandpass filter plate 63 is strictly set, and when the transmission center wavelength is changed by rotating the bandpass filter plate 63 after fixing each of the lenses 61, 62 and optical fibers 52, 54, coupling losses between the optical fibers 52, 54 will increase. It is consequently impossible to construct a tunable bandpass filter by using a fiber-type bandpass filter of this kind.
Due to the factors described above, it has been found that a structure in which a bandpass filter is provided at the emission side of a fiber optic amplifier and in which the bandpass filter and light-receiving module are further coupled by means of another optical fiber, gain of the fiber optic amplifier is substantially reduced and it is difficult to produce a tunable bandpass filter.